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Uniform Tellurium Doping in Black Phosphorus Single Crystals by Chemical Vapor Transport Ziming Zhang, Muhammad Khurram, Zhaojian Sun, and Qingfeng Yan* Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: Doping has been a reliable way to improve the properties of black phosphorus (BP). However, a uniform and large amount of doping in BP remains a challenge. Herein, the synthesis of tellurium-doped black phosphorus (Te-doped BP) single crystals with high crystalline quality is achieved by the chemical vapor transport reaction method. The synthetic route enables a uniform and relatively large amount (up to 0.5% atomic ratio) of Te-doping in BP single crystals. The electrocatalytic oxygen evolution reaction (OER) properties of few-layer Te-doped BP nanosheets prepared by liquid exfoliation were also investigated for the first time. Electrochemical tests demonstrated that the OER onsetpotential of undoped and Te-doped BP nanosheets was 1.63 and 1.49 V, respectively. The result implies that doping provides an effective route to enhance the electrochemical OER performance of BP.

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INTRODUCTION Since 2014, two-dimensional black phosphorus (BP) has created a resurgent interest because of its comprehensive fascinating properties,1 especially its layer-dependent direct band gap (Eg: 0.3−2.0 eV),2,3 excellent room-temperature hole mobility (>103 cm2 V−1 s−1),4,5 and self-biodegradation.6,7 These outstanding properties imply that BP is not only promising in electronic, photovoltaic, and optoelectronic nanodevices8−10 but also suitable for the development of batteries, supercapacitors, and biomedical applications.11−13 Similar to graphene and other two-dimensional materials, BP has attracted tremendous interests from physicists, engineers, chemists, biologists, and material scientists. Recently, in order to improve the properties of BP, doping some chemical elements such as potassium (K), arsenic (As), selenium (Se), and tellurium (Te) into BP single crystals has been demonstrated through different methods.14−20 Among these, the K atoms were sprinkled on top of BP by means of the in situ surface doping technique. The K-doped BP changed from a semiconductor to a band-inverted semimetal.14 The As atoms could be rationally doped into BP crystals with highly tunable chemical compositions by using the chemical vapor transport (CVT) reaction method.17 Furthermore, the Asdoped BP kept the characteristics of a semiconductor with tunable electronic and optical properties.17 The Se atoms could also be doped into BP crystals via the CVT reaction method.18 Significantly, the Se-doped BP had a great improvement on its photoelectrical properties when used for a photodetector. The external quantum efficiency was improved remarkably from 149% to 2993% accompanied by the responsivity increased from 0.77 A W−1 to 15.33 A W−1, which was over 20-fold enhancement than the undoped BP. The pioneer work for the synthesis of Te-doped BP crystals was done by the Liu group, © XXXX American Chemical Society

who successfully doped the Te with a doping level of 0.1% atomic ratio into BP through the super-high-pressure method.20 Although the BP doped with Te could boost the field-effect transport performances (room-temperature hole mobility up to 1850 cm2 V−1 s−1) and ambient stability of BP field-effect transistors (room-temperature hole mobility > 200 cm2 V−1 s−1, after 21 days of ambient exposure), the large distribution in hole mobilities and ON/OFF current ratios of the Te-doped BP field-effect transistors was apparent.20 There are two major factors that might contribute to this phenomenon. One possible reason is that the synthesized Te-doped BP crystals exhibited nonuniform distribution of Te dopant, which was actually evidenced by the Raman spectra collected from randomly selected ground powders of Te-doped BP crystals.20 Another possible reason is the low amount of Te dopant in BP. Although they have tried to increase the Te doping level to 0.5% atomic ratio, unfortunately, it was not successful. The low amount of Te might further amplify the effect of nonuniform distribution of Te dopant on the performance uniformity of Tedoped BP nanodevices. It seems that the existing super-highpressure synthetic strategy not only limits the Te-doping level but also cannot realize the uniform Te-doping. Therefore, a new synthetic strategy for Te-doped BP single crystals is highly desired. Herein, we demonstrate that high-quality Te-doped BP single crystals can be synthesized by using the CVT reaction method. The red phosphorus (RP), tin (Sn), and tellurium(IV) iodide (TeI4) were employed as starting materials in our synthetic strategy. Unlike the previously reported super-highpressure route,20 uniform Te doping in the P atomic lattice of Received: January 31, 2018

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DOI: 10.1021/acs.inorgchem.8b00278 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

which is ultrasonicated at a temperature below 300 K for 24 h. After the ultrasonication, the obtained turbid liquid was centrifuged for 20 min at 3000 rpm to remove the residual bulk Te-doped BP. An organic membrane with a pore size of 100 nm in diameter (Beijing Shenghemo Science and Development Center) was used for the filtration of the top half of the supernatant. The filter residue was dried at 373 K in a vacuum drying oven for further experiments. Undoped BP nanosheets were also prepared for comparison by using the above-mentioned procedure. Catalyst ink was prepared by mixing 5 mg of catalyst (undoped or Te-doped BP single crystals) with 1 mg of carbon black (powder, Sigma-Aldrich) in 1 mL of ethanol (Analytical Reagent, Sinopharm Chemical Reagent Co., Ltd.) and 15 μL of Nafion solution (5 wt %). Then, the above solution was ultrasonicated for 5 min in order to form a homogeneous dispersion. A certain volume of catalyst ink (approximately 25 μg of catalyst) was deposited on a glassy carbon electrode. After that, a uniform catalyst loading (about 0.02 mg/cm2) was achieved after drying gently in ambient condition. Electrocatalytic Oxygen Evolution Reaction (OER) Measurements. The OER performances of few-layer undoped and Te-doped BP nanosheets were investigated by using an electrochemical workstation (CHI600E, CH Instruments, Inc., Shanghai, China) at room temperature. Electrochemical tests were performed in 1 mol/L KOH electrolyte with a standard three-electrode system that consisted of a glassy carbon working electrode, a counter electrode (Pt wire, Tianjin Aidahengsheng Science and Development Co., Ltd.), and a reference electrode (Ag/AgCl, Tianjin Aidahengsheng Science and Development Co., Ltd.). An electrochemical workstation was used to control the bias potential and record the current density variations. The linear sweep voltammograms of both samples were measured at a scan rate of 10 mV s−1.

BP crystals was realized. More impressively, a high Te doping concentration up to 0.5% atomic ratio was achieved, which is the highest doping level reported so far. The crystalline quality, band gap, and environmental stability of the as-grown Tedoped BP single crystals were investigated by a series of structural and optical characterizations. Specifically, the asgrown 0.3% atomic ratio Te-doped BP bulk single crystals have an Eg of 0.308 eV via Tauc Plot calculations. More importantly, the electrocatalytic properties of Te-doped BP were examined by exploring the oxygen evolution reaction (OER) based on few-layer Te-doped BP nanosheets for the first time. The OER onset-potential of undoped and Te-doped BP nanosheets is 1.63 and 1.49 V, respectively. Compared to the undoped BP nanosheets, the electrochemical activity of Te-doped BP nanosheets has been greatly improved.

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EXPERIMENTAL SECTION

Growth of Single Crystals. (1) Red phosphorus (RP, 500 mg, 99.999+%, lump, Alfa Aesar), tin (Sn, 25 mg, 99.999%, granule, Alfa Aesar), and tellurium(IV) iodide (TeI4, 30 mg, 99.9+%, crystalline, Alfa Aesar) were employed as starting materials to synthesize the bulk Te-doped BP single crystals. They were sealed in an evacuated silica glass ampule (p = 0.1 Pa). The ampule was horizontally placed in a tube furnace with an independent heating zone (MTI KJ OTF− 1200X−S, Hefei, China). The starting materials were put at one side of the ampule and located at the hot zone (T1) of the furnace. The hot zone (T1) was heated from room temperature to 870 K in 120 min, and then kept for 90 min at 870 K. Meanwhile, the temperature gradient of the ampule has been determined by using an external thermocouple. It was shown that a temperature gradient between the hot zone (T1) and the cold zone (T2) of ampule was about 200 K. After that, the ampule was cooled to ambient temperature through a natural cooling-off process. Finally, the as-grown Te-doped BP single crystals were obtained. (2) The undoped BP single crystals were synthesized using our recently reported method.21 Synthesis of Few-Layer Te-Doped BP Nanosheets. The amount of Te-doped BP single crystals (100 mg) was sealed in a conical flask (50 mL) with 20 mL of N-methyl-2-pyrrolidone (NMP, Analytical Reagent, Sinopharm Chemical Reagent Co., Ltd.), which is ultrasonicated at a temperature below 300 K for 24 h. After the ultrasonication, the obtained turbid liquid was centrifuged for 20 min at 3000 rpm to remove the residual bulk Te-doped BP. The next step was to obtain a homogeneous dispersion of Te-doped BP nanosheets after being centrifuged for 20 min at 6000 rpm. After that, the powders containing Te-doped BP nanosheets were retrieved, and then they were washed for three times using acetone and ethanol. Finally, they were dispersed in acetone for further characterizations. Characterizations of Te-Doped BP Single Crystals. The layered feature and the chemical compositions of Te-doped BP single crystals were identified by using a scanning electron microscope (SEM) (Hitachi SU8010, Japan) equipped with an energy dispersive X-ray spectroscope (EDX). The valences of P and Te elements were analyzed by using X-ray photoelectron spectra (XPS) (Escalab 250Xi, U.K.). The crystal system of Te-doped BP single crystals was determined by using X-ray diffraction (XRD) (Bruker D8 Advance, Germany) with Cu Kα radiation (λ = 1.5406 Å) at 40 kV and 40 mA. The detailed nanostructures of few-layer Te-doped BP nanosheets were examined by field emission transmission electron microscopy (FETEM, JEOL JEM 2100F, Japan). The micro-Raman spectra were collected via a high-resolution confocal Raman system (Horiba Jobin Yvon LabRAM HR Evolution, France) equipped with a 532 nm laser source. The IR transmission spectra were collected via a microarea IR system (V70/HYPERION 1000, USA). Optical microscope (OM) images were taken by a microscope with a CCD camera (Olympus BX51, China) in ambient atmosphere. Preparation of Catalysts.22 The amount of Te-doped BP single crystals (200 mg) was sealed in a conical flask (100 mL) with 40 mL of NMP (Analytical Reagent, Sinopharm Chemical Reagent Co., Ltd.),

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RESULTS AND DISCUSSION To synthesize the Te-doped BP single crystals, a tube furnace with an independent heating zone was employed. The solid starting materials including RP, Sn, and TeI4 were sealed in a silica ampule and located in the heating site at a specific temperature (T = 870 K), and then kept at that temperature for 1.5 h. In this condition, the temperature gradient within the ampule has been determined by using an external thermocouple close to the hot zone and cold zone of the ampule and found a temperature gradient of about 200 K (Figure 1a). After that, the silica ampule was cooled to ambient temperature through a natural cooling process. The detailed synthesis process can be found in the Experimental Section. Finally, the Te-doped BP single crystals were obtained (Figure 1b). The scanning electron microscope (SEM) images of Te-doped BP crystals demonstrate a well-layered feature (Figure 1c,d). The success of Te doping in the as-grown BP single crystals was demonstrated by using X-ray photoelectron spectroscopy (XPS) measurements, as shown in Figure 2. The as-grown BP single crystals have the P 2p3/2 and 2p1/2 peaks that located at 130.2 and 131.0 eV, respectively, which is the characteristic of pure BP (Figure 2a).23 Other sub-bands of P element are not apparent. According to this evidence, the formation of any other P element-based compounds can be eliminated out. During the process of XPS analysis, the Te 3d5/2 peak locating at 573.4 eV is also revealed (Figure 2b). It is a clear and convincing evidence that the synthesized single crystals are Tedoped BP. Furthermore, the Te doping concentration of about 0.3% atomic ratio was acquired based on the XPS data. To verify that the Te atoms homogeneously distribute in the Te-doped BP crystals, the energy dispersive X-ray (EDX) element mapping measurements were employed by selecting a surface area of the as-grown Te-doped BP single crystal at random, as shown in Figure 3a−c. The result demonstrates the Te element is of homogeneous distribution. In addition, the B

DOI: 10.1021/acs.inorgchem.8b00278 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. (a) SEM image of the as-grown Te-doped BP single crystal. (b) EDX mapping of P element and (c) EDX mapping of Te element, corresponding to the SEM image in (a). (d) EDX spectrum, corresponding to the SEM image in (a). Figure 1. Synthesis of Te-doped BP single crystals via CVT reaction in RP/Sn/TeI4 system. (a) General scheme of the experimental setup for CVT reaction. (b) A photograph of the Te-doped BP single crystals; the grid scale is 1 mm. (c, d) SEM images of the as-grown Te-doped BP single crystals.

Figure 2. XPS characterizations of the as-grown Te-doped BP single crystals. (a) P 2p and (b) Te 3d XPS spectra of the Te-doped BP single crystals.

EDX spectrum (Figure 3d) shows the mass ratio of BP to Te is about 82:1, which means the Te doping concentration of Tedoped BP single crystals is about 0.3% atomic ratio. This result agrees well with the mass ratio of RP and Te precursors as 83:1. As can be seen, the result also agrees well with the XPS characterization. At present, the highest Te-doping level achieved is about 0.5% atomic ratio, but the crystal size is of tens of microns and the yield of Te-doped BP crystals is relatively low, as shown in Figure S1 (Supporting Information). This result demonstrates that the amount of Te dopant directly affects the crystal size and the yield of Te-doped BP crystals. To further verify that the Te atoms homogeneously dope into the P atomic lattice of BP crystals, the X-ray diffraction (XRD) measurements were performed to study the structure of the as-grown undoped and Te-doped BP single crystals. In the XRD measurements, an aluminum (Al) foil was used for calibration of peak position, as shown in Figure 4a. The characteristic diffraction peak of Al foil is 44.64°, corresponding to the (200) plane. The XRD characterizations of the as-grown undoped and Te-doped BP single crystals show only diffraction peaks from the (020), (040), and (060) planes without other

Figure 4. (a) XRD characterizations of the as-grown undoped and Tedoped BP single crystals. (b) Enlarged diffraction peaks of Al (200) plane. (c) Enlarged diffraction peaks of undoped and Te-doped BP (060) plane.

crystalline phase residue (Figure 4a), revealing that both the undoped and the Te-doped BP single crystals belong to the orthorhombic system.24 Furthermore, this result indicates that the Te-doped BP single crystals possess high crystalline quality. As a reference, enlarged diffraction peaks of the Al (200) plane are shown in Figure 4b, which locate at the identical position for both the undoped and the Te-doped BP single crystals. However, the shift of peak position toward lower angle owing to Te doping is evident as can be seen in Figure 4c, which shows the enlarged diffraction peaks of the undoped and Tedoped BP (060) plane. In addition, two other enlarged regions of undoped and Te-doped BP (020) and (040) planes have the same consequence, as shown in Figure S2a,b (Supporting Information). C

DOI: 10.1021/acs.inorgchem.8b00278 Inorg. Chem. XXXX, XXX, XXX−XXX

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were characterized by high-resolution transmission electron microscopy (HRTEM). The few-layer Te-doped BP nanosheets were prepared via the liquid-phase exfoliation technique.27 The detailed preparation process can be found in the Experimental Section. From the HRTEM image (Figure 6a), the d-spacing of this crystal plane is about 0.22 nm (inset of Figure 6a) and perfectly matches the d002.28

Figure 5a shows the characteristic Raman peaks of undoped BP single crystals locate at 362.4, 438.9, and 467.1 cm−1 (black

Figure 6. (a) HRTEM image of the few-layer Te-doped BP nanosheets. The inset is an enlarged HRTEM region. (b) SAED pattern of the few-layer Te-doped BP nanosheet.

We then conducted selected-area electron diffraction (SAED) study to evaluate the more detailed structure of the Te-doped BP single crystals. SAED characterization shows the similar pattern with that of the few-layer undoped BP nanosheets.29 The orthorhombic system characteristic of the Te-doped BP single crystals was again confirmed by SAED scanning (Figure 6b), which matches the results of XRD characterizations well. In addition, the environmental stability of Te-doped BP nanosheets (the thickness between 6 and 12 nm) was studied upon ambient exposure (light illumination, the humidity ranging from 25% to 30%, and the temperature ranging from 298 to 303 K). The degradation process was in situ observed through an optical microscope (OM), as shown in Figure 7 and Figure S3 (Supporting Information). It clearly shows that some small bubbles appeared above the Te-doped BP nanosheets after ambient exposure for 6 min. Although it was reported that Te doping tends to enhance the environmental stability of BP crystals, the above result indicates that the Te-doped BP nanosheets are instable at least at the above-mentioned conditions. The reason for this is that the continuous light illumination might trigger the degradation process of Te-doped BP nanosheets, which is similar to the undoped BP nanosheets.30 Figure 8 illustrates the polarization curves of the few-layer undoped and Te-doped BP nanosheets (0.3% atomic ratio doping) in 1 mol/L KOH electrolyte. The detailed preparation of catalysts and OER measurements can be found in the Experimental Section. It is worth to notice that the few-layer Te-doped BP nanosheets in 1 mol/L KOH exhibit a lower onset potential (about 1.49 V) than the few-layer undoped BP nanosheets (about 1.63 V). Compared to the undoped BP nanosheets, the electrochemical activity of Te-doped BP nanosheets was greatly improved. As the radius and electronegativity of the Te atom are different from those of the P atom, the introduction of Te atoms into the P atomic lattice of BP crystals maybe causes electrons modulation to change the charge distribution and electronic properties of phosphorus skeletons, the same as the heteroatom doping in carbon based materials (such as graphene and carbon nanotubes, etc.).31 Therefore, the Te atoms doping affect the BP nanosheets interaction with oxygen intermediates and ultimately their

Figure 5. Optical characterizations and Eg of the as-grown undoped and Te-doped BP single crystals. (a) Typical Raman spectra and (b) micro-IR spectra of the undoped and Te-doped BP single crystals. (c) The hν−(hνF(R∞))2 curve of the undoped BP single crystals. (d) The hν−(hνF(R∞))2 curve of the Te-doped BP single crystals.

line), corresponding to the vibrations of the A1g, B2g, and A2g phonon modes, respectively.25 Compared to the undoped BP single crystals, the A1g, B2g, and A2g Raman peaks of the Tedoped BP single crystals (0.3% atomic ratio doping) are observed to shift toward lower wavenumber, and the reduced values are 2.4, 2.8, and 3.7 cm−1, respectively. The observed shifts of A1g, B2g, and A2g peaks can be attributed to Te doping. It is speculated that doping induces the changes of the thickness of each layer and the d-spacing between layers in the Te-doped BP single crystals. Note that we tested three regions of the as-grown Te-doped BP single crystal at random, corresponding to the pink line, blue line, and red line, respectively, as shown in Figure 5a. Contrary to the previously observed wide variation of peaks shift,20 the reduced values of the three modes for the randomly selected three regions are similar, which convincingly indicates the uniform distribution of Te dopant inside the crystals. The Eg of the as-grown Te-doped BP single crystals was also investigated by using micro-IR spectroscopy and Tauc Plot calculations.26 Figure 5b shows the undoped and Te-doped BP single crystals have a strong transmittance in the middle IR region. The Eg values of the undoped and Te-doped BP single crystals were estimated based on the relational expression: (hνF(R∞))1/n = A(hν − Eg), where h is the Planck’s constant, ν is the frequency of vibration, A is the proportional constant, and n value is 1/2. The hν−(hνF(R∞))2 curves of the undoped and Te-doped BP single crystals are shown in Figure 5c,d, respectively. The Eg of the Te-doped BP single crystals is 0.308 eV, which is similar to the undoped BP single crystals (Eg = 0.301 eV). In order to further ensure the high crystalline quality of the as-grown Te-doped BP crystals, their detailed nanostructures D

DOI: 10.1021/acs.inorgchem.8b00278 Inorg. Chem. XXXX, XXX, XXX−XXX

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of 0.308 eV. The electrocatalytic OER properties measurement of few-layer undoped and Te-doped BP nanosheets indicates that the electrochemical activity of Te-doped BP nanosheets has been greatly improved in comparison with the undoped one.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00278. EDX characterization of the as-grown Te-doped BP single crystal, XRD characterizations of the as-grown undoped and Te-doped BP single crystals, and OM characterization of Te-doped BP nansheets upon ambient exposure (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Qingfeng Yan: 0000-0002-5084-6886 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 21671115) and the SAMSUNG 2015 Global Research Outreach (GRO) Program.

Figure 7. OM images of Te-doped BP nanosheets upon ambient exposure, taken after ambient exposure for (a) 0 min, (b) 2 min, (c) 4 min, (d) 6 min, (e) 8 min, and (f) 10 min. The scale bar is 10 μm.

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Figure 8. Polarization curves of the few-layer undoped and Te-doped BP nanosheets in 1 mol/L KOH electrolyte.

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CONCLUSIONS In summary, we report for the first time the synthesis of Tedoped BP single crystals by using the CVT reaction method based on the RP, Sn, and TeI4 system. Structural and optical characterizations show that the synthesized Te-doped BP single crystals exhibit high crystalline quality. Most importantly, our synthetic route enables uniform Te atoms doping in the P atomic lattice of BP crystals, which is evidenced by the XRD and Raman shift with a small variation. In addition, a large amount of Te doping up to 0.5% atomic ratio is achieved, which is the highest doping level reported so far. The as-grown 0.3% atomic ratio Te-doped BP bulk single crystals have an Eg E

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DOI: 10.1021/acs.inorgchem.8b00278 Inorg. Chem. XXXX, XXX, XXX−XXX